Clathrin-Coated Vesicles

Core Concept

Clathrin is the coat that buds vesicles from the plasma membrane and the trans-Golgi network (TGN), delivering cargo to endosomes (and, from the PM, carrying material into the cell by endocytosis). Clathrin itself is a structural cage built from triskelions — three-legged molecules of three heavy and three light chains — that do not touch the membrane or cargo directly. Instead, adaptor protein (AP) complexes sit between the clathrin cage and the membrane: they bind the membrane (via lipids), bind clathrin, and select cargo by recognizing sorting signals in the cytoplasmic tails of receptors. Because clathrin-coated buds at the plasma membrane have long, thin necks, their scission requires the GTPase dynamin, which pinches the vesicle off. The coat is then rapidly removed by an ATP-driven uncoating machinery.

Key Components

• Clathrin triskelion: three heavy chains + three light chains; polymerizes into a polyhedral cage.
• AP2 adaptor: at the plasma membrane; binds PI(4,5)P₂, cargo, and clathrin.
• AP1 adaptor: at the TGN; binds cargo and clathrin for TGN → endosome transport.
• Sorting signals: e.g., tyrosine-based YXXΦ and dileucine [DE]XXXL[LI] motifs in cargo tails.
• Dynamin: a GTPase that forms a helix around the vesicle neck and drives scission.
• Hsc70 + auxilin: the ATP-driven uncoating machinery.
• Amphiphysin/BAR proteins: help recruit dynamin and sense/generate neck curvature.

Mechanism / How It Works

1. Adaptor recruitment. AP2 binds the plasma membrane through phosphoinositide (PI(4,5)P₂) and binds the cytoplasmic tails of receptors bearing YXXΦ or dileucine signals.
2. Cage assembly. Clathrin triskelions are recruited by the adaptors and self-assemble into a curved lattice that progressively invaginates the membrane, concentrating cargo.
3. Scission. As the bud deepens into a deep pit with a narrow neck, dynamin assembles as a helix around the neck. GTP hydrolysis drives a conformational change that constricts and severs the neck, releasing the coated vesicle.
4. Uncoating. Auxilin recruits Hsc70, whose ATP hydrolysis disassembles the clathrin cage and releases the adaptors, leaving a naked vesicle that can fuse with an early endosome.
5. Delivery and recycling. The vesicle fuses with an early endosome; receptors and adaptors are recycled back to the plasma membrane or TGN for reuse.

Energy and Directionality

Clathrin transport consumes GTP (dynamin, for scission) and ATP (Hsc70, for uncoating; plus phosphoinositide metabolism upstream). The binding energy of adaptor–cargo and clathrin–adaptor interactions drives cage assembly and curvature, while phosphoinositides spatially direct the whole process (PI(4,5)P₂ marks the plasma membrane, and PI(4)P the TGN). Directionality is set by this lipid-based targeting plus the SNAREs and Rabs that specify the acceptor endosome.

Experimental Evidence / Technique

Clathrin coats were first visualized by electron microscopy of purified vesicles as the characteristic polyhedral "basket" lattice, and triskelions were revealed by rotary shadowing. Dynamin's role in scission was shown by the Drosophila shibire mutant, which at restrictive temperature accumulates unsevered coated pits with dynamin collars at their necks — proving dynamin pinches off the vesicle. In vitro reconstitution with clathrin, AP2, dynamin, and GTP produced buds and, with dynamin, scission. RNAi and knockout studies mapped AP2 cargo signals (YXXΦ, dileucine).

How it works

1. Adaptor recruitment. AP2 binds the plasma membrane through phosphoinositide (PI(4,5)P₂) and binds the cytoplasmic tails of receptors bearing YXXΦ or dileucine signals.
2. Cage assembly. Clathrin triskelions are recruited by the adaptors and self-assemble into a curved lattice that progressively invaginates the membrane, concentrating cargo.
3. Scission. As the bud deepens into a deep pit with a narrow neck, dynamin assembles as a helix around the neck. GTP hydrolysis drives a conformational change that constricts and severs the neck, releasing the coated vesicle.
4. Uncoating. Auxilin recruits Hsc70, whose ATP hydrolysis disassembles the clathrin cage and releases the adaptors, leaving a naked vesicle that can fuse with an early endosome.
5. Delivery and recycling. The vesicle fuses with an early endosome; receptors and adaptors are recycled back to the plasma membrane or TGN for reuse.

Common confusions

• "Clathrin binds cargo directly." — Clathrin is a structural cage; adaptors bind cargo and membrane, then recruit clathrin.
• "Dynamin makes the bud." — Dynamin only severs the neck; the coat/adaptors make the bud.
• "Clathrin only works at the plasma membrane." — It also buds from the TGN (via AP1) toward endosomes/lysosomes.
• "COPI and clathrin use the same scission mechanism." — Clathrin requires dynamin for scission; COPII/COPI scission is coat-driven (no dynamin).
• "All endocytosis is clathrin-mediated." — There are clathrin-independent routes (caveolae, macropinocytosis), but clathrin is the major pathway.

Quick review

• Clathrin = triskelion cage; adaptors (AP2 at PM, AP1 at TGN) select cargo.
• Cargo signals: YXXΦ and dileucine.
• Dynamin (GTP) severs the neck; Hsc70/auxilin (ATP) uncoat.
• Routes: PM → endosome (endocytosis), TGN → endosome.
• LDL/transferrin receptors internalized by this pathway; defects → familial hypercholesterolemia.